1. Field of the Invention
The present invention relates to an optical proximity correction method, an optical proximity correction apparatus, an optical proximity correction program, a method of manufacturing a semiconductor device, a design rule formulating method, and an optical proximity correction condition calculating method.
2. Description of the Related Art
Generally, a number of circuit elements are integrated in semiconductor devices. The most well known of those circuit elements is a transistor having a gate disposed between source and drain regions. Semiconductor devices having such a transistor as one of its components are manufactured by photolithography.
According to photolithography, as a circuit pattern to be formed on a wafer is smaller in size, it becomes more difficult to form a basic design pattern to desired shape and dimensions on the wafer. A process of correcting a mask shape for forming the basic design pattern to desired shape and dimensions on the wafer is called an optical proximity correction (hereinafter referred to as “OPC”) process, which is indispensable in the modern lithographic technology. An OPC process for correcting a mask shape including process conversion differences due to various causes, which may also be called a PPC (Process Proximity Correction) process, is also used in the art.
The OPC process is carried out according to a sequence shown in FIG. 11 of the accompanying drawings. According to the OPC process, the data of a design pattern (e.g., data in GDSII format) and OPC conditions (e.g., calculating conditions for the OPC process) are read into an OPC tool (steps 702, 702, a step will hereinafter be abbreviated as “S”). Thereafter, the outer edge of a layout pattern of a circuit is divided into small pieces called segments (S703). An evaluating point (hereinafter referred to as “EP”) representing a segment is set for each segment (S704). Segment positions on a pattern to be formed on a wafer are calculated by a simulation (S705), after which a variation of the calculated value of each segment from a target value (hereinafter referred to as “edge placement error” (EPE)) is calculate (S706). The shape of a mask pattern is modified, i.e., the segment positions are corrected, in order to keep the EPEs of all segments within specification values (S707, S708). Segment positions are calculated by iterative calculations to determine the shape of the mask pattern. In other words, the segment positions are changed in order to minimize the EPEs. The above OPC process is called model base OPC.
In the process of manufacturing semiconductor devices which have become more and more shrinking in recent years, it is very difficult to form a complete resist pattern according to a design pattern even through the OPC process, and the formed resist pattern tends to deform somewhat from the design pattern. Due to such a pattern deformation, semiconductor devices having a transistor as one of its components may have a transistor gate pattern distorted in two-dimensional directions within a plane. Such a gate pattern distortion greatly affects the electric characteristics of the transistor. It has therefore been proposed to focus attention on the electric characteristics of circuit elements for correcting mask patterns. (See, for example, Japanese laid-open No. 2003-330162, International publication No. 2003/052512 pamphlet, U.S. Pat. Nos. 6,562,638, and 6,775,818, hereinafter referred to as Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4, respectively).
The related art referred to above suffers the following difficulties:
Patent document 1 discloses that electric characteristics of a semiconductor device with a transistor gate pattern distorted two-dimensionally are determined in advance and their correlation to the two-dimensional distortions is stored as a reference table, and that a gate length is determined in order to achieve desired electric characteristics. However, since Patent document 1 is silent about a process of quantitatively expressing two-dimensional gate pattern distortions, it is difficult to apply the disclosed process to actual semiconductor devices having various layouts.
Patent document 2 discloses that after a pattern processed by OPC is simulated by a simulating means to generate a print image of a gate pattern, it is determined whether a variation of the print image of the gate pattern from a design value for a certain gate length falls in an allowable range or not depending on demanded characteristics of a circuit, and if the variation does not fall in the allowable range, the edge of the pattern is shifted. If the demanded characteristics of the circuit are represented by a reduced leak current, then it is determined whether the minimum value of the gate length falls in an allowable range or not. If the demanded characteristics of the circuit are represented by an increased circuit performance, then the maximum value of the gate length is used. However, Patent document 2 fails to sufficiently show a process of setting the allowable range of the gate length. Therefore, the allowable range of the gate length is not related to the demanded characteristics of the circuit.
Patent document 3 reveals that a gate length distribution is extracted from a print image of a gate pattern, and electric characteristics of a circuit device are calculated from the extracted gate length distribution, after which a parametric yield of a semiconductor device is calculated from the distribution of the electric characteristics, and the gate pattern is corrected to keep the yield in a specification range. However, Patent document 3 lacks a specific description about how the pattern is to be corrected according to OPC, and is likely to suffer a heavy calculation load in calculating the electric characteristics of the circuit.
Patent document 4 discloses that depending on the shape of a simulated pattern to be printed to a wafer, electric characteristics of a transistor and wiring characteristics are calculated, and process conditions for fabricating semiconductor devices are corrected depending on those characteristics. However, Patent document 4 contains no description as to various characteristic changes due to two-dimensional printed pattern distortions that have been at issue in recent years. Therefore, the disclosed process is considered to have difficulty dealing with various characteristic changes.